Microelectromechanical system (MEMS) accelerometers are widely used in many different application areas such as geophysical surveying, underwater imaging, navigation, medical, automotive, aerospace, military, tremor sensing, consumer electronics, etc. These sensors typically detect acceleration by measuring the change in position of a proof mass, for example, by a change in the associated capacitance. Traditional capacitive MEMS accelerometers may have poor performance due to low noise suppression and sensitivity, however.
Measurement noise and range may vary for different applications of sensors. For example, for a navigation application, a measurement range of ±20 g may be desired and 1 μg/√Hz measurement noise for this range could be tolerated. As another example, a tremor sensing application may desire a ±1 g measurement range and a lower noise floor of ˜10-100 ng/√Hz. The main type of noise affecting this noise floor is Brownian noise. Brownian noise refers to noise produced by Brownian motion. Brownian motion refers the random movement of particles suspended in a liquid or gas resulting from their bombardment by the fast-moving atoms or molecules in the liquid or gas.
Accelerometers may have many uses in the field of geophysical surveying, particularly marine seismic. For example, in some marine seismic embodiments, a survey vessel may tow one or more streamers in a body of water. Seismic sources may be actuated to cause seismic energy to travel through the water and into the seafloor. The seismic energy may reflect off of the various undersea strata and be detected via sensors on the streamers, and the locations of geophysical formations (e.g., hydrocarbons) may be inferred from these reflections.
These streamer sensors that are configured to receive the seismic energy may include accelerometers such as those described in this disclosure. (Various other sensors may also be included in some embodiments, such as pressure sensors, electromagnetic sensors, etc.)
Additionally, accelerometers may be used to detect the relative positions of the streamers (or portions thereof) via acoustic ranging. Acoustic ranging devices typically may include an ultrasonic transmitter and electronic circuitry configured to cause the transceiver to emit pulses of acoustic energy. The travel time of the acoustic energy between a transmitter and receivers (e.g., accelerometers) disposed at a selected positions on the streamers is related to the distance between the transmitter and the receivers (as well as the acoustic velocity of the water), and so the distances may be inferred.
In other marine seismic embodiments, accelerometers according to this disclosure may also be used in permanent reservoir monitoring (PRM) applications, for example at a seafloor. Generally, the term “geophysical survey apparatus” may refer to streamers, PRM equipment, and/or sensors that form portions of streamers or PRM equipment.
Accordingly, improvements in accelerometer technology (e.g., allowing better performance and/or lower cost) may provide substantial benefits in the geophysical surveying field, among other fields.
This specification includes references to “one embodiment” or “an embodiment.” The appearances of the phrases “in one embodiment” or “in an embodiment” do not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure.
Various units, circuits, or other components may be described or claimed as “configured to” perform a task or tasks. In such contexts, “configured to” is used to connote structure by indicating that the units/circuits/components include structure (e.g., circuitry) that performs the task or tasks during operation. As such, the unit/circuit/component can be said to be configured to perform the task even when the specified unit/circuit/component is not currently operational (e.g., is not on). The units/circuits/components used with the “configured to” language include hardware—for example, circuits, memory storing program instructions executable to implement the operation, etc. Reciting that a unit/circuit/component is “configured to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. §112, sixth paragraph, for that unit/circuit/component.